I had to get an MRI today as a result of a car accident, and I was shocked at how incredibly loud it is while in the mangeto-tube. It sounded, and felt like, several jackhammers were going off all around me. Nothing like being in a confined space, strapped down, and having it sound like the entire machine is going to collapse around you.

An MRI depends on creating an alternating magnetic field -- a really, really hugely powerful one. That field affects anything that can be attracted by a magnet (which is why they're really careful about what kinds of metals you can take in there with you) and even if it's fastened down securely, it's going to rattle back and forth as the magnetic field changes. There really isn't any way to prevent that.posted by Class Goat at 1:55 PM on October 29, 2008

Oh, also, the coil that's producing the field is going to rattle as it interacts with the Earth's magnetic field.posted by Class Goat at 1:56 PM on October 29, 2008 [1 favorite]

I'm familiar with the concept of the MRI and how it works -- quite amazing science. It is hard to believe that the noises that I heard were random bits of magnetically active objects bouncing about due to the field; the frequency of the vibrations were constant and on the order 1-3 Hz.posted by virga at 2:11 PM on October 29, 2008

To quote the literature I was given when I went to the MRI center in town for the first time "The noise is due to the rising electrical current in the wires of the gradient magnets being opposed by the main magnetic field. The stronger the main field, the louder the gradient noise. " This is a pretty good explanation.

They're loud. It's a side effect of how they work. Changing the magnetic field causes a buildup of energy, and lot of the energy is released as soundwaves (some of it's released as heat, which is why there's some pretty impressive cooling systems in use on the machines as well).posted by jlkr at 2:21 PM on October 29, 2008

Hard to believe, but true.

All structures have what is known as a "first harmonic frequency" (or "critical frequency", FHF hereafter) - bridges, buildings, and MRIs. In tuning forks & church bells, their FHF serve a useful purpose (and are pretty highly isolated; that is, most of the energy goes straight into that frequency).

In the case of structures like the MRI, the design goal is to move the FHF as high as possible above the input vibrations, or else the Tacoma Bridge Collapse happens. In your car, if it's too close to the vibrations from the bumps on the road, the car "bounces" rhythmically. All of this is bad.

However, the reality is that the vibrations aren't completely restricted to frequencies above the FHF. Below there, input vibrations transfer some energy, but at much reduced levels. Nonetheless, if a gigawatt goes in, and 0.001% is transferred to low-F parts of the spectrum, that's still a lot of energy resonating!

If you stand near a power transformer (at relay stations, or those canisters hanging from power poles), you can hear the 60 Hz hum - it's the magnetic coils in the transformers, leeching useful power into vibration, and ultimately waste heat.posted by IAmBroom at 2:24 PM on October 29, 2008 [1 favorite]

Most of it is the vibration caused by the electromagnets turning on and off, as Class Goat said. The RF coils are generally right next to your head, so you get to hear the effect up close and personal.

However, I don't believe that the earth's magnetic field has an appreciable effect on the noise, since MRI magnet strengths are around 100,000 times as powerful as the earth's magnetic field. Other contributors to general noise include the coolant pumping system.

The reason that the period of the banging sounds was about 2 seconds is because the scanner was zapping you every 2 seconds. A strong magnetic field is applied to the tissue and what is measured is the fall-off in the magnetization of the tissue after the field is lowered.posted by demiurge at 2:29 PM on October 29, 2008

Like everyone else is saying, it really is just that noisy to make magnets produce incredibly detailed images of your innards.

I like to write ambient industrial songs to the changing rhythms, whirs, and bangs I hear during my MRIs. Since I've got a few more coming up, I'm hoping I can eventually get a whole album out of it.posted by batmonkey at 2:32 PM on October 29, 2008 [2 favorites]

With the possible exception of jlkr so far, nobody in this thread has provided an appropriate answer, and jlkr's link is messed up so I can't tell if it was useful or not.

This is a question I'd also love to know the answer to, but so far everyone is saying things like "That's just the way it works." or "That's how it takes pictures!"

Sound is the result of air pressure differences interacting with your ear drum. Let's work backwards from that - what in the MRI is moving the air? What is causing that thing to move?posted by odinsdream at 3:01 PM on October 29, 2008

Glad you asked this and I'll be watching for answers too. My memories of my MRIs the sounds remided me of various bangings and one period bangs that had the rapidity of machine gun fire, and it was LOUD.posted by X4ster at 3:10 PM on October 29, 2008

I can't speak much to the science, but as to the vibrations being of a regular and specific frequency - the sounds the machine makes are dependent on the exact MRI sequence they used on you. I used to work in an MRI research lab, and we would use three or four different magnetic sequences in a session, each making a completely different awful loud rhythmic noise. Some slow, some fast, some high-pitched, some low-pitched, all coming from the same machine with the only different thing being the specific magnetic field sequence we chose.

They were all horrible, though.posted by Stacey at 3:31 PM on October 29, 2008

The main magnetic field, which is essentially constant in strength (1.5-3.0 Teslas) and direction, is produced by a large, typically superconducting magnet. The field gradient magnets produce a varying magnetic field only a few percent of that strength in order to select the position of the slice of the image, and they are the source of most of the noise, which is produced as their coils expand and contract from Lorentz forces as they interact with the field of the larger magnet. According to the Wikipedia article, the sounds can reach 120 db (!), which might actually be louder than a jackhammer.

Fairly strong criticisms of the claim that the Tacoma Narrows bridge collapsed due to a resonance have been made over the last couple of decades:

Physics textbooks and instructors usually attribute the bridge's collapse to the phenomenon of resonance. Like a mass hanging from a spring, a suspension bridge oscillates at a natural frequency. In the case of the Tacoma Narrows bridge, so the explanation goes, the wind blowing past the bridge generated a train of vortexes that produced a fluctuating force in tune with the bridge's natural frequency, steadily increasing the amplitude of its oscillations until the bridge finally failed.

That explanation, however, is incomplete and flawed. Engineers Robert H. Scanlan of Johns Hopkins University in Baltimore and K. Yusuf Billah of Princeton University sought to set the record straight by providing a detailed account of what actually went wrong and why in an article in the February, 1991 American Journal of Physics.
...The standard engineering explanations, however, haven't satisfied applied mathematician P. Joseph McKenna of the University of Connecticut. He has spent nearly two decades developing alternative mathematical models to account for the undulations and gyrations shown by the ill-fated bridge. What distinguishes suspension bridges is their fundamental nonlinearity, McKenna argues. ...posted by jamjam at 3:40 PM on October 29, 2008

Oooops?

Must have been a slip of the ctrl+V fingers. Is this better? looks better on my end, at least.posted by jlkr at 3:45 PM on October 29, 2008

If you're asking how the coils end up producing sound as a result of the electromagnetism, the answer is that they vibrate. The electromagnetic field that they're generating produces forces within the coil which cause it to move. Their movement moves the air, which is sound.

The question is really the same as "why do transformers hum." If you understand why a transformer hums, when it's got a constant stream of 60Hz running through it, then it's fairly easy to work up to why the MRI makes all sorts of banging/clunking sounds, since it is working with pulses of current at very high (RF) frequencies.

Scientists in Switzerland have explained, for the first time, the microscopic cause of hum in those massive yokes of iron which help transform AC electricity from one voltage to another.

When current reverses 60 times a second the iron core of the transformer undergoes magnetetostriction twice during each cycle. In other words, 120 times per second induced fields cause the core to stretch slightly; a meter-sized transformer might stretch or shrink by only a micron but this would be enough to set up an audible 120-Hz hum. [...]

The new experimental work probes theories, going all the way back to Werner Heisenberg in the 1920s, about how the shrinkage arises from the magnetic interactions (spin exchange) among pairs of atoms (dimers), which share a common electron. The two magnetic ions want to be closer together.

When standing next to a 60Hz transformer, you actually hear the sound produced by the movement of the transformer (or by metal parts being moved by the field that surrounds it, which I suspect may play a large role as well, but I've no proof of this). In the case of an MRI machine, since the fundamental frequency of the current going through the coils is up in the RF, you're only hearing either low-frequency harmonics of the fundamental frequency, or sound being produced by the initial on and off edge of the pulse through the coil as a whole. (I.e., when current starts going through the coil it moves slightly, when the current gets cut off and the field collapses, it moves again slightly ... those movements become sound.)

If you had a recording of the sound done with a good microphone, you could probably figure out what was producing it....harmonics or the pulse itself. My bet is on the pulse itself.

Does that clear anything up? Hopefully I haven't just muddied the waters further...posted by Kadin2048 at 3:55 PM on October 29, 2008

Thanks, jamjam. In retrospect, my answer was a bit vague, and wandering. I concede the geek cred point to you.

You haven't seen the last of me yet... We shall meet again, my friend! [poof]posted by IAmBroom at 4:23 PM on October 29, 2008

I'm sure you'll have little difficulty sweeping me from the field, IAmBroom; my only hope is that you may be deterred by the prospect of sticky bristles.posted by jamjam at 4:42 PM on October 29, 2008

We have a winner! Awesome information, thanks!posted by odinsdream at 4:44 PM on October 29, 2008

I tend to disbelieve that it is the magnetic field causing any sort of noise -- I work with superconducting magnets every day, and the PDF dmd linked to points out many other sources of noise in the MRI room -- pumps, air handling, equipment, etc. I do not think the main source is the magnet itself. In fact, we had a power outage yesterday (the superconducting magnet is unaffected by this because it is a static field i.e. not plugged in to the wall) and it was dead silent in the instrument room. If anything within the magnet is causing the noise it would be the gradient coils that PDF refers to -- those actually have currents running through them that can be controlled. The static magnet, however, was totally silent (several magnets ranging from 7-14 T, although this is quite a bit higher than what is used for MRI's)

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